| 1 | // Copyright 2022 The Go Authors. All rights reserved. |
|---|---|
| 2 | // Use of this source code is governed by a BSD-style |
| 3 | // license that can be found in the LICENSE file. |
| 4 | |
| 5 | package typeparams |
| 6 | |
| 7 | import ( |
| 8 | "go/types" |
| 9 | ) |
| 10 | |
| 11 | // CoreType returns the core type of T or nil if T does not have a core type. |
| 12 | // |
| 13 | // See https://go.dev/ref/spec#Core_types for the definition of a core type. |
| 14 | func CoreType(T types.Type) types.Type { |
| 15 | U := T.Underlying() |
| 16 | if _, ok := U.(*types.Interface); !ok { |
| 17 | return U // for non-interface types, |
| 18 | } |
| 19 | |
| 20 | terms, err := _NormalTerms(U) |
| 21 | if len(terms) == 0 || err != nil { |
| 22 | // len(terms) -> empty type set of interface. |
| 23 | // err != nil => U is invalid, exceeds complexity bounds, or has an empty type set. |
| 24 | return nil // no core type. |
| 25 | } |
| 26 | |
| 27 | U = terms[0].Type().Underlying() |
| 28 | var identical int // i in [0,identical) => Identical(U, terms[i].Type().Underlying()) |
| 29 | for identical = 1; identical < len(terms); identical++ { |
| 30 | if !types.Identical(U, terms[identical].Type().Underlying()) { |
| 31 | break |
| 32 | } |
| 33 | } |
| 34 | |
| 35 | if identical == len(terms) { |
| 36 | // https://go.dev/ref/spec#Core_types |
| 37 | // "There is a single type U which is the underlying type of all types in the type set of T" |
| 38 | return U |
| 39 | } |
| 40 | ch, ok := U.(*types.Chan) |
| 41 | if !ok { |
| 42 | return nil // no core type as identical < len(terms) and U is not a channel. |
| 43 | } |
| 44 | // https://go.dev/ref/spec#Core_types |
| 45 | // "the type chan E if T contains only bidirectional channels, or the type chan<- E or |
| 46 | // <-chan E depending on the direction of the directional channels present." |
| 47 | for chans := identical; chans < len(terms); chans++ { |
| 48 | curr, ok := terms[chans].Type().Underlying().(*types.Chan) |
| 49 | if !ok { |
| 50 | return nil |
| 51 | } |
| 52 | if !types.Identical(ch.Elem(), curr.Elem()) { |
| 53 | return nil // channel elements are not identical. |
| 54 | } |
| 55 | if ch.Dir() == types.SendRecv { |
| 56 | // ch is bidirectional. We can safely always use curr's direction. |
| 57 | ch = curr |
| 58 | } else if curr.Dir() != types.SendRecv && ch.Dir() != curr.Dir() { |
| 59 | // ch and curr are not bidirectional and not the same direction. |
| 60 | return nil |
| 61 | } |
| 62 | } |
| 63 | return ch |
| 64 | } |
| 65 | |
| 66 | // _NormalTerms returns a slice of terms representing the normalized structural |
| 67 | // type restrictions of a type, if any. |
| 68 | // |
| 69 | // For all types other than *types.TypeParam, *types.Interface, and |
| 70 | // *types.Union, this is just a single term with Tilde() == false and |
| 71 | // Type() == typ. For *types.TypeParam, *types.Interface, and *types.Union, see |
| 72 | // below. |
| 73 | // |
| 74 | // Structural type restrictions of a type parameter are created via |
| 75 | // non-interface types embedded in its constraint interface (directly, or via a |
| 76 | // chain of interface embeddings). For example, in the declaration type |
| 77 | // T[P interface{~int; m()}] int the structural restriction of the type |
| 78 | // parameter P is ~int. |
| 79 | // |
| 80 | // With interface embedding and unions, the specification of structural type |
| 81 | // restrictions may be arbitrarily complex. For example, consider the |
| 82 | // following: |
| 83 | // |
| 84 | // type A interface{ ~string|~[]byte } |
| 85 | // |
| 86 | // type B interface{ int|string } |
| 87 | // |
| 88 | // type C interface { ~string|~int } |
| 89 | // |
| 90 | // type T[P interface{ A|B; C }] int |
| 91 | // |
| 92 | // In this example, the structural type restriction of P is ~string|int: A|B |
| 93 | // expands to ~string|~[]byte|int|string, which reduces to ~string|~[]byte|int, |
| 94 | // which when intersected with C (~string|~int) yields ~string|int. |
| 95 | // |
| 96 | // _NormalTerms computes these expansions and reductions, producing a |
| 97 | // "normalized" form of the embeddings. A structural restriction is normalized |
| 98 | // if it is a single union containing no interface terms, and is minimal in the |
| 99 | // sense that removing any term changes the set of types satisfying the |
| 100 | // constraint. It is left as a proof for the reader that, modulo sorting, there |
| 101 | // is exactly one such normalized form. |
| 102 | // |
| 103 | // Because the minimal representation always takes this form, _NormalTerms |
| 104 | // returns a slice of tilde terms corresponding to the terms of the union in |
| 105 | // the normalized structural restriction. An error is returned if the type is |
| 106 | // invalid, exceeds complexity bounds, or has an empty type set. In the latter |
| 107 | // case, _NormalTerms returns ErrEmptyTypeSet. |
| 108 | // |
| 109 | // _NormalTerms makes no guarantees about the order of terms, except that it |
| 110 | // is deterministic. |
| 111 | func _NormalTerms(typ types.Type) ([]*Term, error) { |
| 112 | switch typ := typ.(type) { |
| 113 | case *TypeParam: |
| 114 | return StructuralTerms(typ) |
| 115 | case *Union: |
| 116 | return UnionTermSet(typ) |
| 117 | case *types.Interface: |
| 118 | return InterfaceTermSet(typ) |
| 119 | default: |
| 120 | return []*Term{NewTerm(false, typ)}, nil |
| 121 | } |
| 122 | } |
| 123 |
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